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Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

This document describes a language tag for use in cases where it is desired
to indicate the language used in an information object, how to register values for use
in this language tag, and a construct for matching such language tags, including user
defined extensions for private interchange.

1. Introduction

Human beings on our planet have, past and present, used a number of languages.
There are many reasons why one would want to identify the language used when presenting
or requesting information.

Information about a user's language preferences commonly needs to be identified
so that appropriate processing can be applied. For example, the user's language
preferences in a brower can be used to select web pages appropriately. A choice
of language preference can also be used to select among tools (such as
dictionaries) to assist in the processing or understanding of content in
different langauges.

In addition, knowledge about the particular language used by some piece of information content may be
useful or even required by some types of information processing; for example
spell-checking, computer-synthesized speech, Braille transcription, or
high-quality print renderings.

One means of indicating the language used is by labeling the information content
with a language identifier. These
identifiers can also be used to specify user preferences when selecting information content,
or for labeling additional attributes of content and associated resources.

These identifiers can also be used to indicate additional attributes of
content that are closely related to the language. In particular, it is
often necessary to indicate specific information about the dialect,
writing system, or orthography used in a document or resource, as these
attributes may be important for the user to obtain information in a form
that they can understand, or important in selecting appropriate
processing resources for the given content.

This document specifies an identifier mechanism, a registration function for values
to be used with that identifier mechanism, and a construct for matching against those
values. It also defines a mechanism for private use extension and how private use,
registered values, and matching interact.

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in [RFC 2119][10].

2. The Language Tag

The language tag is composed of one or more parts: A primary language subtag and
a (possibly empty) series of subsequent subtags. Subtags are distinguished by
their length and content, so that each type of subtag can be recognized solely
by these features. This makes it possible to construct a parser that can extract and
assign some semantic information to the subtags, even if specific subtag values are
not recognized.

The tags and their subtags, including private use extensions, are to be treated
as case insensitive: there exist conventions for the
capitalization of some of them, but these should not be taken to carry meaning. For
instance, [ISO 3166][4] recommends that country codes be capitalized (MN Mongolia),
while [ISO 639][3] recommends that language codes be written in lower case (mn Mongolian).
In the language tags defined by this document, however, the tag 'mn-MN' is not
distinct from 'MN-mn' or 'mN-Mn' (or any other combination) and each of these
variations conveys the same meaning.

For examples of language tags, see Appendix B at the end of this document.

The namespace of language tags and their subtags is administered by the
Internet Assigned Numbers Authority (IANA) [15] according
to the rules in Section 3.1.

Terminology in this section:

Tag or tags refers to a complete language tag, such as 'fr-Latn-CA'

Subtag refers to a specific section of a tag, separated by hyphen, such as
the subtag 'Latn' in 'fr-Latn-CA'

Code or codes refers to tags defined in external standards (and which are used
as subtags in this document). For example,
'Latn' is an ISO 15924[2] script code (which
can be used as a script subtag in a language tag)

The rules in this section apply to the various subtags within the language tags
defined in this document, excepting those "grandfathered" tags defined in
Section 2.2.1.

Note that registered subtags can only appear in specific positions in a tag.
Specifically, they can only occur as primary (language) subtags, as extended
language subtags delmited with the single letter subtag "s" or as variant
subtags.

In addition, private use subtags may only occur at the end of the
sequence of subtags and will not be interspersed with subtags defined in the
rules that follow.

Each subtag type has unique length and content restrictions that make
identification of the subtag's type possible, even if the content of
the subtag itself is unrecognized. This allows tags to be parsed and processed without reference
to the latest version of the underlying standards or the IANA registry and
it makes the associated exception handling when parsing tags simpler.

Single letter and digit subtags are reserved for current or future use. These include the following current uses:

The single letter subtag "i" is reserved for use with grandfathered
IANA registrations that begin with that tag, such as "i-hakka".
Language tags or subtags with the "i" subtag prefix MUST NOT be registered
in the future. That is, the list of tags or subtags identified by the
subtag "i-" will not change in the future.

The single letter subtag "x" is reserved to introduce a sequence
of private use (or "extension") subtags. No subtags defined by the
rules in this section or in any standard or registry defined in this
document follow the "x" subtag.

The single letter subtag "s" is reserved to introduce a language
extension subtag. Language extension subtags are currently reserved
for future standardization.

All other single-character subtags are reserved and many only be used
by revision of this document.

The primary subtag is the first subtag in a language tag and cannot be empty.
Except as noted, the primary subtag is the "language" subtag. The following
rules apply to the assignment and interpretation of the primary subtag:

All 2-character subtags are interpreted according to assignments
found in ISO standard 639, "Code for the representation of names of languages"
[ISO639-1][3], or assignments subsequently made
by the ISO 639 Part 1 maintenance agency or governing standardization bodies.

All 3-character subtags are interpreted according to assignments
found in ISO 639 part 2, "Codes for the representation of names of languages --
Part 2: Alpha-3 code [ISO 639-2][1]", or assignments subsequently made by the ISO 639
part 2 maintenance agency or governing standardization bodies, or assignments
of 3-character disambiguation registrations according to Rule 7a. Ambiguity in
Section 2.3 of this document.

ISO639-2 reserves for private use codes the range 'qaa' through 'qtz'. These
codes should be used for non-registered language subtags.

Subtags of 5 to 15 characters may be registered with IANA, according to the
rules in Section 3.1 of this document. (Note that
previously, in rfc3066[16],
the IANA registry contained whole tag registrations such as 'de-CH-1994',
whereas this document refers to the registration of subtags such as 'tsolyani')

The single character subtag "x" as the primary subtag indicates that the
whole language tag is a private use tag. The value and semantic meaning of
such a tag as a whole and of the subtags used within such as tag are undefined
by this document.

The single character subtag "i" as the primary subtag indicates one of
the grandfathered IANA registered tags starting with "i", such as
"i-tsolyani"

Other values shall not be assigned to the primary subtag except by
revision of this document.

The following rules apply to the extended language subtags:

Each extended language subtag must be prefixed with the single letter
subtag "s". If there are two or more extended language subtags, each
such subtag must be preceded by the subtag "s".

Extended language subtags are currently reserved for future
standardization.

Extended language subtags must follow the primary subtag, precede
any other subtags, and there may be more than one extended language
subtag.

Note: The order of the extended language subtags is important in some
interpretations of language tags. See the section Section 2.4.

Example: 'zh-s-min-s-nan' would represent the subdialect 'nan' of the
Chinese dialect 'min'.

The following rules apply to the script subtags:

All 4-character subtags are interpreted as ISO 15924 alpha-4 script codes from
[2], or subsequently assigned by the ISO 15924 maintenance agency or
governing standardization bodies, denoting the script or writing system used in
conjunction with this language. These alpha4 tags may only occur following
the primary language subtag and any extended language subtags and must occur
before any other type of subtag described below.

Example: 'de-Latn' represents German written using the Latin script.

ISO 15924 reserves the codes Qaaa-Qacz for private use values. These codes
should be used for non-registered script values.

Script subtags MUST NOT be registered using the process in
Section 3.1 of this document. Variant subtags may be
considered for registration for that purpose.

The following rules apply to the region subtags:

The region subtag must follow any language, extended language, or script
subtags and must precede all other subtags.

All 2-character subtags following the primary subtag denote
the region or area to which this language variant relates, and are
interpreted according to assignments found in ISO 3166 alpha-2 country
codes from [4], assignments subsequently made
by the ISO 3166 maintenance agency, or governing standardization bodies.

All 3-character codes consisting of digit (numeric) characters denote
the region or area to which this language tag relates, and are interpreted
according to the assignments found in UN Standard
Country or Area Codes for Statistical Use[5] or assignments made thereto
by the governing standards body.

The ISO3166 code MUST be used to form the
subtag except for countries with ambiguous ISO 3166 alpha-2 codes
as defined in Rule 7a in Section 2.3.

If the ISO 3166 alpha-2 code for a country is ambiguous as defined
in Rule 7a, then the UN numeric code MUST be used to form the region
subtag.

UN numeric codes assigned to 'macro-geographical (continental)' or
sub-regions not associated with an assigned ISO3166 alpha-2 code MAY be
used to form the region subtag.

UN numeric codes for 'economic groupings' or 'other groupings' MUST NOT
be used to form language tags.

Note: Generally there will be an informative IANA registration for valid
UN numeric country codes. Continental codes will not be listed in the
IANA registry, but may be used.

Note: the alphanumeric codes in Appendix X of the UN document
must not be used. (At the time this document was created these values
match the ISO 3166 alpha-2 codes.)

ISO 3166 reserves the country codes AA, QM-QZ, XA-XZ and ZZ as user-assigned codes.
These codes should be used for private use region subtags.

Region subtags MUST NOT be registered using the process in Section 3.1 of
this document. Variant subtags may be considered for registration for this
purpose.

Region subtags must occur after any script subtags and before any variant
subtags or extensions.

Example: 'de-Latn-CH' represents German written using Latin
script for Switzerland.

Variant subtags must follow all of the other defined subtags, but
precede any private use extensions.

Alphanumeric subtags of 5 to 15 characters may be registered with
IANA, according to the rules in Section 3.1 of this
document.

Registered subtags MUST NOT begin with the character 'x', which
is reserved for private use subtags. (Note that previously, in rfc3066,
the IANA registry contained whole tag registrations such as 'en-boont',
whereas this document refers to the registration of subtags such as 'boont')

Alphanumeric subtags of 5 to 15 characters starting with 'x'
are reserved for private use. The semantics of these subtags must be defined
by the end users of such subtags and the semantic meaning should be considered
external to this document.

The following rules apply to private use extensions:

Private Use Extensions subtags are separated from the other subtags
defined in this document by the reserved single-character subtag "x".

Private Use Extension subtags must consist of one to thirty-two alphanumeric
characters, with each subtag separated by a single "-".

No source is defined for private use extensions. Use or standardization
of the private use extension subtags is by private agreement and should
not be considered part of this document.

For example: Users who wished to utilize SIL Ethonologue for identification might
agree to exchange tags such as 'az-Arab-x-AZE-derbend'. This
example contains two extension subtags. The first is "AZE" and the second is
"derbend".

Existing IANA registered language tags from RFC1766/RFC3066 that are not defined
by additions to this document maintain their validity. IANA will maintain these tags,
adding a notation that they are "grandfathered from RFC 3066".

If the formerly registered tag would now be defined by this document, then the
existing tag is marked as superseded by this document and no subtag will be
registered as a result. For example, 'zh-Hans' is now defined by the addition
of ISO 15924 script codes.

If the registered tag contained one or more subtags that follow the guidelines
for registered language or variant subtags, and all of the subtags are either
now defined by this document or would be valid to register, then each subtag not
already covered by this document will be registered automatically by IANA without
further review and the existing tag marked as superseded by this document.
For example: the tag 'en-boont' fits the pattern for a registered variant.
The variant subtag "boont" will be registered automatically and 'en-boont'
marked as superseded.

If the registered tag contains any subtags that are not otherwise valid for
registration according to the rules in this document, then the tag as a whole
is maintained as an exceptional case (that is, it is "grandfathered"). This
includes special cases of Sign Language tags. For example, the tag 'i-enochian' is
not covered by any addition and is grandfathered, as is
'sgn-BE-fr' (Belgian French Sign Language).

Users of tags that are grandfathered should consider registering appropriate
subtags using the new format (but are not required to).

Languages not listed in ISO 639 that are not variants of any listed language,
can be registered, such as i-mingo. Before attempting to register a language
subtag, there should be a good faith attempt to register the language with ISO 639.
No language subtags will be registered for codes that exist in ISO 639-1 or ISO 639-2.

Dialect or other divisions or variations within a language, its orthography,
writing system, regional variation, or historical usage, such as the "scouse"
subtag (the Scouse dialect of English).

This document leaves the decision on what subtags are appropriate or not to the
registration process described in Section 3.1.

ISO 639 defines a maintenance agency for additions to and changes in the list of
languages in ISO 639. This agency is:

One may occasionally be faced with several possible tags for the same body of text.

Interoperability is best served if all users send the same tag, and use the same tag
for the same language for all documents. If an application has requirements that make
the rules here inapplicable, the application protocol specification MUST specify
how the procedure varies from the one given here.

The text below is based on the set of tags known to the tagging entity.

Use as precise a tag as possible, but no more specific than is justified.
For example, 'de' might suffice for tagging an email written in German,
while 'de-CH-1996' is probably unnecessarily precise for such a task.

Avoid using subtags that add no distinguishing information about the content.
For example, the script subtag in 'en-Latn-US' is generally unnecessary, since
nearly all English texts are written in the Latin script.

When a language has both an ISO 639-1 2-character code and an ISO 639-2 3-character
code, you MUST use the ISO 639-1 2-character code.

When a language has no ISO 639-1 2-character code, and the ISO 639- 2/T (Terminology)
code and the ISO 639-2/B (Bibliographic) codes differ, you MUST use the Terminology code.
NOTE: At present all languages that have both kinds of 3-character code also are assigned
a 2-character code, and the displeasure of developers about the existence of two different
code sets has been adequately communicated to ISO. So this situation will hopefully
not arise.

You SHOULD NOT use the UND (Undetermined) code unless the protocol in use forces you
to give a value for the language tag, even if the language is unknown. Omitting the tag
is preferred.

You SHOULD NOT use the MUL (Multiple) tag if the protocol allows you to use
multiple languages, as is the case for the Content-Language header in HTTP.

NOTE: In order to avoid versioning difficulties in applications such as that
experienced in RFC 1766[8], the ISO 639 Registration Authority Joint Advisory
Committee (RA-JAC) has agreed on the following policy statement:

"After the publication of ISO/DIS 639-1 as an International Standard, no new
2-letter code shall be added to ISO 639-1 unless a 3-letter code is also added
at the same time to ISO 639-2. In addition, no language with a 3-letter code available at the time of publication of ISO 639-1 which at that time had no 2-letter code shall be subsequently given a 2-letter code."

This will ensure that, for example, a user who implements "haw" (Hawaiian),
which currently has no 2-character code, will not find his or her data invalidated
by eventual addition of a 2-character code for that language."

To maintain backwards compatibility, there are two provisions to account for potential
instability in ISO 639, 3166, and 15924 codes.

a) Ambiguity.

Beginning with these standards as of
1 January, 2003, in the event that ISO639, ISO3166, or ISO15924 assigns a
code a new meaning or reassigns a deprecated code, the new use of the code is
not permitted in language tags defined by this document.

In the event that either ISO 639 or ISO 15924 assigns a new meaning to
an existing code, the language subtag reviewer, as described in Section 3,
shall prepare a proposal for entering in the IANA registry as soon as
practical a variant or registered language subtag as a surrogate value for
the new code. The form of the registered language subtag or variant subtag
will be at the discretion of the language subtag reviewer and must conform
to other restrictions on language or variant subtags in this document.

In the event ISO 3166 assigns a new meaning to an existing code, then the
language subtag reviewer, as described in Section 3, shall prepare a proposal
for entering the appropriate numeric UN country code as an informative entry in the
IANA registry.

The normal registration process described in
Section 3.1 of this document applies to the review and
registration of the registered subtags described above. Note that these
subtags should never be used in combination with the subtag type for which
they are a surrogate. For example, a "region" variant subtag should not be
used with a region subtag.

For example:

cs-CS (Czech for Czechoslovakia)

sr-891 (Serbian for Serbia and Montenegro, using the UN country code)

qx-Latn (hypothetical reassigned value 'qx')

qx2003-Latn (hypothetical registered language subtag)

b) Stability.

All other ISO codes are valid, even if they have been
deprecated. Some examples, current at the time this document was drafted, are
listed below. Where a new equivalent code has been defined (given below on the
right side after a tilde), implementations should treat these tags as identical.

The language tag always defines a language as spoken (or written, signed or
otherwise signaled) by human beings for communication of information to other human
beings. Computer languages such as programming languages are explicitly excluded.

If a language tag B contains language tag A as a prefix, then B is typically
"narrower" or "more specific" than A. For example, 'zh-Hant-TW' is more specific
than 'zh-Hant'.

This relationship is not guaranteed in all cases: specifically,
languages that begin with the same sequence of subtags are NOT guaranteed to be
mutually intelligible, although they may be. For example, the tag 'az'
shares a prefix with both 'az-Latn' (Azerbaijani written using the Latin script) and
'az-Cyrl' (Azerbaijani written using the Cyrillic script). A person fluent in one script
may not be able to read the other, even though the text might be identical. Content
tagged as 'az' most probably is written in just one script and thus might
not be intelligible to a reader familiar with the other script.

The relationship between the tag and the information it relates to is defined by
the standard describing the context in which it appears. Accordingly, this section
can only give possible examples of its usage.

For a single information object, it could be taken as the set of languages
that is required for a complete comprehension of the complete object.
Example: Plain text documents.

For an aggregation of information objects, it should be taken as the set of
languages used inside components of that aggregation. Examples: Document stores
and libraries.

For information objects whose purpose is to provide alternatives, the set of
tags associated with it should be regarded as a hint that the content is provided
in several languages, and that one has to inspect each of the alternatives in order
to find its language or languages. In this case, a tag with multiple languages
does not mean that one needs to be multi-lingual to get complete understanding of
the document. Example: MIME multipart/alternative.

In markup languages, such as HTML and XML, language information can be added to
each part of the document identified by the markup structure (including the whole
document itself). For example, one could write
<span lang="FR">C'est la vie.</span> inside a Norwegian document;
the Norwegian-speaking user could then access a French-Norwegian dictionary
to find out what the marked section meant. If the user were listening to that
document through a speech synthesis interface, this formation could be used
to signal the synthesizer to appropriately apply French text-to-speech
pronunciation rules to that span of text, instead of misapplying the
Norwegian rules.

A Language Range is a set of languages whose tags all begin with the same
sequence of subtags. The following definition of language-range is derived from
HTTP/1.1[14].

language-range = language-tag / "*"

That is, a language-range has the same syntax as a language-tag, or is the
single character "*" and implicitly assumes that there is a semantic relationship
between tags that share the same subtag prefixes.

A language-range matches a language-tag if it exactly equals the tag, or
if it exactly equals a prefix of the tag such that the first character following
the prefix is "-".

The special range "*" matches any tag. A protocol which uses language ranges
may specify additional rules about the semantics of "*"; for instance, HTTP/1.1
specifies that the range "*" matches only languages not matched by any other
range within an "Accept-Language:" header.

As noted above, not all languages
or content denoted by a specific language-range may be mutually intelligible and
this use of a prefix matching rule does not imply that language tags are assigned
to languages in such a way that it is always true that if a user understands a
language with a certain tag, then this user will also understand all languages
with tags for which this tag is a prefix. The prefix rule simply allows the use
of prefix tags if this is the case.

Implementations that are searching for content or otherwise matching language
tags to a language-range [Section 2.4.1] may choose to assume that
there is a semantic relationship between two tags that
share common prefixes. This is called 'language tag fallback'. The most
common implementations follow this pattern:

When searching for content using language tag fallback, the language tag is
progressively truncated from the end until a match is located. For example,
starting with the tag 'en-US-boont', searchs or matches would first be performed with the
whole tag, then with 'en-US', and finally with 'en'. This allows some flexibility
in finding content in accordance with Rules 1 and 2 in Section 2.3;
allows better maintenance; and usually provides better
results when data is not available at a specific level of tag granularity or is
sparsely populated (than if the default language for the system or content
were used). Any implementation that uses this technique should ensure that
appropriate data is available on each level.

Tag to match: en-US-boont
1. en-US-boont
2. en-US
3. en

Default Fallback Pattern Example

Private Use Extensions are orthogonal to language tag fallback. By default,
implementations should ignore private use extensions and follow the default
fallback pattern (above). Thus matching the tag "en-US-boont-x-traditional" would
be exactly the same as the example above.

Implementations that choose to interpret one or more private use extension subtags
can choose a different fallback pattern or use the private use extensions to
interpret content in a different fashion.

3. IANA Considerations

The procedure given here MUST be used by anyone who wants to use a subtag not
given an interpretation in Section 2.2 of this document or previously
registered with IANA.

This procedure MAY also be used to register information with the IANA about a
tag or subtag defined by this document, for instance if one wishes to make publicly
available a reference to the definition for a language such as
sgn-US (American Sign Language), or additional information about a registration
previously made via this procedure.

Variant subtags MUST NOT be registered using the pattern 2 ALPHA * DIGIT to
accommodate the provisions in Section 2.3, rule 7a of this document. That is,
the subtag yx1234 can NOT be registered except under the aforementioned provisions.

Extended language subtags that are registered MUST be at least 4 characters long.

Subtags MUST NOT be registered that start with the letter 'x', since
this prefix is reserved for Private Use subtags.

The process starts by filling out the registration form reproduced below.

LANGUAGE SUBTAG REGISTRATION FORM
Name of requester:
E-mail address of requester:
Subtag to be registered:
Type of Subtag:
[ ] language
[ ] extended language
[ ] variant
[ ] region (informative; for use by language subtag reviewer only)
Full English name of subtag:
Intended meaning of the subtag:
If variant subtag, the intended prefix(es) of subtag:
If extended language subtag, the intended prefix(es) of subtag:
Native name of language (transcribed into ASCII):
Reference to published description of the language (book or article):
Any other relevant information:

The subtag registration form MUST be sent to <ietf-languages@iana.org> for
a two week review period before it can be submitted to IANA. (This is an open list.
Requests to be added should be sent to <ietf-languages-request@iana.org>.)

Variant subtags are generally registered for use with a particular prefix or
set of prefixes. For example, the subtag 'boont' is intended for use with the
prefix 'en-', since Boontling is a dialect of English. This information MUST be
provided in the registration form.

Any registered subtag MAY be incorporated into a variety of language tags,
according to the rules of Section 2.1. This makes validation
simpler and thus more uniform across implementations, and does not require new
registrations for different intended prefixes.

However, the intended prefixes for a given registered subtag will be maintained
in the IANA registry as a guide to usage. If it is necessary to add an additional
intended prefix to that list for an existing language tag, that can be done by filing
an additional registration form. In that form, the "Any other relevant information: "
field should indicate that it is the addition of an additional intended prefix.

When the two week period has passed, the subtag reviewer, who is appointed by the
IETF Applications Area Director, either forwards the request to IANA@IANA.ORG, or
rejects it because of significant objections raised on the list. Note that the
reviewer can raise objections on the list himself, if he or she so desires. The
important thing is that the objection must be made publicly.

The applicant is free to modify a rejected application with additional information
and submit it again; this restarts the two week comment period.

Updates of registrations follow the same procedure as registrations. The subtag
reviewer decides whether to allow a new registrant to update a registration made
by someone else; normally objections by the original registrant would carry extra
weight in such a decision.

Registrations are permanent and stable. When some registered subtag should not be
used any more, for instance because a corresponding ISO 639 code has been created,
the registration should be amended by adding a remark like
"DEPRECATED: use <new code> instead" to the "other relevant information" section.

Note: The purpose of the "published description" is intended as an aid to people
trying to verify whether a language is registered, or what language a particular
subtag refers to. In most cases, reference to an authoritative grammar or dictionary
of that language will be useful; in cases where no such work exists, other well
known works describing that language or in that language may be appropriate. The
subtag reviewer decides what constitutes "good enough" reference material.

4. Security Considerations

The only security issue that has been raised with language tags since the publication
of RFC 1766, which stated that "Security issues are believed to be irrelevant to
this memo", is a concern with language ranges used in content negotiation - that
they may be used to infer the nationality of the sender, and thus identify potential
targets for surveillance.

This is a special case of the general problem that anything you send is visible
to the receiving party. It is useful to be aware that such concerns can exist in
some cases.

The evaluation of the exact magnitude of the threat, and any possible countermeasures,
is left to each application protocol.

5. Character Set Considerations

Language tags may always be presented using the characters A-Z, a-z, 0-9, and HYPHEN-MINUS,
which are present in most character
sets, so presentation of language tags should not have any character set issues.

The issue of deciding upon the rendering of a character set based on the
language tag is not addressed in this memo; however, it is thought impossible
to make such a decision correctly for all cases unless means of switching language
in the middle of a text are defined (for example, a rendering engine that decides
font based on Japanese or Chinese language may produce sub-optimal output when a
mixed Japanese- Chinese text is encountered)

6. Changes from RFC3066

The main goals were to maintain backward compatibility (so that all
previous codes would remain valid); reduce the need for large numbers of
registrations; to provide a more formal structure to allow parsing into subtags
even where software does not have the latest registrations; to provide stability
in the face of potential instability in ISO 639, 3166, and 15924 codes
(demonstrated instability in the case of ISO 3166); and to allow for
external extension mechanisms.

Allows ISO15924 script code subtags and allows them to be used generatively.

Adds the concept of a variant subtag and allows variants to be used generatively.

Adds an extension mechanism which does not require registration to use.

Defines the private use tags in ISO639, ISO15924, and ISO3166 as the mechanism
for creating private use language, script, and region subtags respectively

Defines a syntax for private use variant subtags which can be used without
registration.

Defines a process for handling reuse of values by ISO639, ISO15924, and ISO3166
in the event that they register a previously used value for a new purpose.

Changes the IANA language tag registry to a language subtag registry

Substantive changes between draft-01 and this version are:

Added a reference to the most recent version of the UN country IDs to
the address information in section 2.2.2.

Removed references to the 'i-klingon' tags (previously used as examples)
since that tag is now deprecated (due to the addition of the ISO639-2
tag 'tlh').

Made the choice of UN or ISO3166 codes explicit
in Section 2.2 and modified the text of Rule 7a. It also sets a start date for
ambiguity resolution.

Prohibited future registration of "i-" prefixed tags or subtags.

Extensive non-substantive edits were made to the text to clarify positioning
and make the rules for subtag assignment clearer.

Statistical Division, United Nations, "Standard Country or Area Codes for Statistical Use", UN Standard Country or Area Codes for Statistical Use, Revision 4 (United Nations publication, Sales No. 98.XVII.9, June 1999.

Authors' Addresses

Appendix A. Acknowledgements

Any list of contributors is bound to be incomplete; please regard the following as only a selection from the group of people who have contributed to make this document what it is today.

The contributors to RFC 3066 and RFC 1766, the precursors of this document, made enormous contributions directly or indirectly to this document and are generally responsible for the success of language tags.

The following people (in alphabetical order) contributed to this document or to RFCs 1766 and 3066:

Very special thanks must go to Harald Tveit Alvestrand, who originated RFCs 1766 and 3066, and without whom this document would not have been possible. Special thanks must go to Michael Everson, who has served as language tag reviewer for almost the complete period since the publication of RFC 1766.

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